Low risk from major accident consequences

02 February 2012

A severe accident at a US nuclear power plant would not be likely to cause any immediate deaths, while the risks of fatal cancers caused by such an accident would be millions of times lower than the general risks of dying of cancer, a long-running research study has found.

"The analyzed accidents would cause essentially zero immediate deaths and only a very, very small increase in the risk of long-term cancer deaths."

Nuclear Regulatory Commission

The US Nuclear Regulatory Commission (NRC) launched the State-of-the-Art Reactor Consequence Analyses (SOARCA) research project in 2007, and a draft report has now been completed and opened to public comment. Focusing on the Surry and Peach Bottom nuclear power plants as representative of pressurised water reactors (PWRs) and boiling water reactors (BWRs) operational in the USA, the in-depth study combines up-to-date information on plant layout and operations, local population data and emergency preparedness plans with state-of-the-art computational analysis tools and best modelling practices. These have been used to provide an evaluation of accident progression and offsite consequences for a selection of severe accident scenarios which the authors say are more realistic than previous analyses.

Scenarios considered for both plants included short- and long-term station blackouts, involving the loss of all alternating current power. Scenarios including a containment bypass involving the rupture of steam generator pipes were also considered for the Surry PWR plant. Such scenarios would not be relevant in a BWR plant, which does not utilise steam generators.

SOARCA's main conclusions fall into three areas: how a reactor accident progresses; how existing systems and emergency measures can affect an accident's outcome; and how an accident would affect the public's health.

According to the report, the studies have shown that existing resources and procedures can stop an accident, slow it down or reduce its impact before it can affect the public, but even if accidents proceed without such mitigation they take much longer to happen and release much less radioactive material than earlier analyses suggested. Moreover, the analysed accidents would cause "essentially zero immediate deaths and only a very, very small increase in the risk of long-term cancer deaths". Latent cancer fatality risk from the selected specific scenarios was found to be thousands of times lower than the NRC's own so-called Safety Goal and millions of times lower than the general cancer fatality risk in the United States from all causes, even when employing the linear no-threshold (LNT) dose-response model, which assumes that health risk is directly proportional to radiation exposure and that even the smallest radiation exposure carries some risk.

The US study was already nearing completion when the severe accident at Japan's Fukushima Daiichi nuclear power plant took place in March 2011. The Fukushima accident had both similarities and differences with the SOARCA severe accident scenarios for Peach Bottom, and these are considered in an appendix to the report based on the current extent of knowledge on events at Fukushima.

The SOARCA results, while specific to Peach Bottom and Surry, may be generally applicable to plants with similar designs, the report notes, although cautioning that additional work would be needed to confirm this because of differences in plant-specific designs, procedures, and emergency response.

Seismic updates

The Electric Power Research Institute, US Department of Energy and US Nuclear Regulatory Commission have released a new seismic study to help nuclear facilities in the central and eastern USA to reassess seismic hazards. The report is the culmination of a four-year exercise and replaces previous seismic source models in use by industry and government since the 1980s.

The model can be used to calculate the likelihood of various levels of ground motions caused by earthquakes, based on a data set covering over 400 years of historical and geological data for the entire region.